Idiopathic thrombocytopenic purpura, the most common acquired hemorrhagic disorder, is characterized by platelet destruction due to antiplatelet autoantibodies. Treatment is empirical, based on general measures of immunosuppression or reticuloendothelial blockade. We have sought to determine the basis for the disorder, establish diagnostic and prognostic serologic tests, and develop innovative therapy. Using strategies of cloning cDNA fragments of a major platelet membrane glycoprotein target of ITP antibodies (GPIb); expressing the respective protein fragments; synthesizing peptides of high antigenic indices within reactive fragments; and epitope scanning with overlapping 15-mer peptides, we have localized a chief epitope on GPIb to the sequence TKEQTTFPP. This sequence has partial homology to 15 known proteins, but none appear to be reasonable cross-reactive immunogens. Soluble GP peptides or protein fragments we studied were not effective competitors for ITP Abs that attach to intact platelets, but may be valuable solid-phase adsorbents in therapeutic pheresia procedures. Applications of flow cytometry, antigen capture radioimmunoassays, and newly developed electrochemiluminescence immunoassays to the detection of plasma ITP Abs indicate the Abs are too low in affinity or concentration to be measured effectively for clinical utility. In contrast, antibody eluates of patients' platelets or concentrated indirect eluates prepared with their plasma are reliable sources of potent Ab for diagnostic and possibly prognostic applications. The recently discovered stimulator of megakaryocyte proliferation and maturation, thrombopoietin (TPO), was found, in collaboration with R. Cohen at Genentech, who developed an ELISA system for this ligand, to be below normal levels in sera of 18 ITP patients with severe thrombocytopenia. This indicates that TPO may be regulated by megakaryocyte mass rather than platelet number and that it should be considered for trials as a therapy for ITP.